urpi

Lack of sleep and stress also increase memory problems and raise the risk to fall into depression.

Just use some simple memory tests, now I think they could be very good indicators of performance. Under extreme stress I begun forgetting things, for eg. short term memory for numbers was almost completely gone.

Some non-direct measurements are just daily behaviour; do you lose your temper easier or do you just try get isolated.

Been there, done that. I would rather pay attention on just mental health and daily physical exercise than mental performance, things will normalize within a year anyway when the kid grows up and sleeps more steadily.